The present invention relates to a semiconductor memory device, and more particularly to a semiconductor memory device stably operating over a wide range of the power supply voltage.
Currently, operating voltages of semiconductor memory devices are being diversified and lowered. Using a low power supply voltage is generally advantageous in decreasing the power consumption, especially in a semiconductor memory device which employs a battery as a power supply. However, the low power supply voltage causes various problems. For example, the low power supply voltage may not be high enough to drive word lines. This is because, in order to fully turn on a cell transistor, the voltage for driving the word lines must be higher than the voltage for driving the bit lines. Taken a DRAM cell as an example; when data "1" in the low power supply voltage level is stored in a storage capacitor via a bit line pair by turning on a cell transistor, it is preferable to boost the potential level of the data "1" up to the power supply voltage plus the threshold voltage of the cell transistor, prior to storing the data. To this end, a boost voltage is employed, which is obtained by boosting the power supply voltage up to a predetermined level, using a boosting circuit. Generally, a boosting ratio indicative of how many times the power supply voltage is boosted up from the power supply voltage, is fixed. Consequently, the boost voltage resulting from a overly-low power supply voltage is still too low to overcome the threshold voltage, thereby causing a malfunction during writing in and/or reading out the data. Contrarily, the boost voltage resulting from a overly-high power supply voltage becomes excessively high to thus destroy a gate layer of the cell transistor, thereby causing an inevitable damage to the memory device.
A technique is suggested for solving the foregoing problems in Symposium on VLSI Circuits, 1990, by Nakagome et al. in an article entitled: "A 1.5 V Circuit Technology for 64 Mb DRAMs". The article discloses that if a power supply voltage in a range of 1.5 V-3.3 V is applied to the semiconductor memory device, the power supply voltage is converted into 1.5 V to drive the memory device. Accordingly, the memory device can be operated by the power supply voltage of a wide voltage range, e.g., 1.5 V-3.3 V. In addition, since the power supply voltage is lowered to a voltage of 1.5 V in the memory device to be used as an operating voltage, the power consumption is reduced. In this technique, to drive the word lines, the voltage of 1.5 V is boosted up to a predetermined level by a boosting circuit (refer to 1990 Symposium on VLSI Circuits Digest of Technical Papers, pp. 17-18, for more detailed technical information).
However, since the relatively low voltage of 1.5 V is used in the above described conventional technique, the boosting ratio should be high enough to obtain a sufficiently boosted voltage for driving the word lines. Consequently, the boosting circuit should operate very frequently to boost up the power supply voltage, which causes an increase of the power consumption.
Furthermore, since a sufficiently high power supply voltage is forcibly dropped down to be used as an operating voltage of 1.5 V, the power is unnecessarily consumed. Also, with using pump capacitors, to boost up the voltage which has been forcibly dropped down to 1.5 V is very inefficient, and brings on a power loss caused by operations of the boosting circuit.